Cherry D M, Amy N K
Department of Nutritional Sciences, University of California, Berkeley 94720.
J Nutr. 1987 Dec;117(12):2054-60. doi: 10.1093/jn/117.12.2054.
Rat liver xanthine oxidase activity is regulated in response to dietary protein and iron. To investigate whether the change in activity was mediated by a change in the rate of protein degradation, we measured the fractional turnover rate using the double-isotope technique with [3H]- and [14C]leucine and calculated the apparent half-life of xanthine oxidase in rats fed diets containing either 20 or 5% casein with either 35 or 5 mg iron/kg diet. Under control conditions, xanthine oxidase had an apparent half-life of 4.8 d and approximately 65% of the enzyme subunits were active. Rats fed diets with low dietary protein had lower xanthine oxidase activity, but the enzyme had a slower fractional turnover rate, resulting in an apparent half-life of 6.4 d, and only 15-20% of the enzyme was active. The apparent half-life of xanthine oxidase increased to 7.5 d in rats fed diets with low dietary iron, but dietary iron did not affect the specific activity of the enzyme or the percentage of active subunits. These results suggest that the loss of enzyme activity is not due to loss of enzyme protein by increased degradation, but rather to inactivation of the enzyme.
大鼠肝脏黄嘌呤氧化酶的活性受膳食蛋白质和铁的调节。为了研究活性变化是否由蛋白质降解速率的改变介导，我们采用双同位素技术，用[³H] - 和[¹⁴C]亮氨酸测量了分数周转率，并计算了在含有20%或5%酪蛋白以及每千克饲料含35毫克或5毫克铁的饲料喂养的大鼠中黄嘌呤氧化酶的表观半衰期。在对照条件下，黄嘌呤氧化酶的表观半衰期为4.8天，约65%的酶亚基具有活性。喂食低蛋白饲料的大鼠黄嘌呤氧化酶活性较低，但该酶的分数周转率较慢，导致表观半衰期为6.4天，且只有15 - 20%的酶具有活性。在喂食低铁饲料的大鼠中，黄嘌呤氧化酶的表观半衰期增加到7.5天，但膳食铁不影响该酶的比活性或活性亚基的百分比。这些结果表明，酶活性的丧失不是由于降解增加导致酶蛋白的丢失，而是由于酶的失活。
